How can you effectively threat model legacy systems?

1 Assess the current state

Before you can threat model a legacy system, you need to have a clear understanding of its current state. This includes the architecture, components, dependencies, data flows, functionality, and security controls of the system. You also need to know the context and environment in which the system operates, such as the business objectives, regulatory requirements, user expectations, and threat landscape. You can use various tools and techniques to collect and document this information, such as interviews, surveys, audits, code reviews, diagrams, and checklists.

2 Identify and prioritize threats

Once you have a comprehensive view of the legacy system, you can start identifying the potential threats that could compromise its confidentiality, integrity, availability, or functionality. You can use frameworks and methodologies such as STRIDE, DREAD, or OWASP Top 10 to guide your threat identification process. You can also leverage existing sources of threat intelligence, such as vulnerability databases, security reports, or industry standards. After you have a list of threats, you need to prioritize them based on their likelihood and impact, and assign them a risk rating or score.

3 Mitigate and monitor threats

The next step is to determine how to mitigate and monitor the threats that you have identified and prioritized. You can use the risk rating or score as a criterion to decide which threats need immediate attention, which ones can be accepted or transferred, and which ones can be deferred or ignored. You can also use the principle of defense in depth to apply multiple layers of security controls to protect the legacy system, such as encryption, authentication, authorization, logging, auditing, patching, testing, and backup. You should also establish a process for monitoring and reporting on the threats and their mitigation status, and for updating the threat model as the system or the environment changes.

4 Communicate and collaborate

Threat modeling is not a one-time or isolated activity. It requires constant communication and collaboration among various stakeholders, such as developers, testers, operators, managers, and users. You should involve them throughout the threat modeling process, from the initial assessment to the final mitigation and monitoring. You should also share and document the results and recommendations of the threat modeling process, and solicit feedback and suggestions for improvement. By communicating and collaborating effectively, you can ensure that everyone is aware of the risks and responsibilities associated with the legacy system, and that they can contribute to its security and resilience.

5 Learn and improve

Threat modeling is also a learning and improvement opportunity. You can use the threat modeling process to identify and address the root causes of the security issues and vulnerabilities in the legacy system, and to prevent them from recurring in the future. You can also use the threat modeling process to identify and implement the best practices and standards for security design and development, and to align them with the business and user needs. You can also use the threat modeling process to measure and evaluate the effectiveness and efficiency of your security controls and processes, and to identify and implement the areas for improvement.

Threat modeling legacy systems can be challenging, but also rewarding. By following these tips and best practices, you can enhance the security and resilience of your legacy systems, and reduce the risks they pose to your organization.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?